The present invention generally relates to a resistor and more particularly, to a high-voltage variable resistor, for example, a focusing variable resistor for use in a television set.
Although conventionally, for example, a flyback transformer for use with the high-voltage variable resistor has been especially designed to suppress its voltage regulation and to minimize variations in the width of pictures on a screen of a television set, such voltage regulations may alternatively be restricted to a small value by causing a large current to flow through the high-voltage variable resistor for supplying focusing voltage and screen voltage to the cathode-ray tube of the television set.
A table 1 below shows one example of relations of total resistance RT of the high-voltage variable resistor connected to the flyback transformer, variations in width of pictures on the screen, and voltage regulation of the flyback transformer.
TABLE 1 ______________________________________ Total resistance RT (M.OMEGA.) 100 75 67 50 Variations in width of 8 6 5 3 pictures on screen (mm) Voltage regulation (KV) 3.75 3.60 3.55 3.35 ______________________________________
It is to be noted that voltage applied to the high-voltage variable resistor in Table 1 is approximately 13.5 KV. It is easily understood from Table 1 that characteristics of the flyback transformer can be remarkably improved by reducing the total resistance RT. Meanwhile, reduction in the total resistance RT will increase load for the high-voltage variable resistor, thus raising temperature of the high-voltage variable resistor by Joule's heat.
Conventionally, in high-voltage variable resistors, it has been so arranged as shown in FIGS. 1(a) to 1(d) that connecting terminal pins 3a for receiving a high voltage supplied from a flyback transformer, 3b for supplying a focusing voltage, 3c for supplying a screen voltage, and 3d for grounding or earthing are soldered to an insulating substrate 1 at right angles thereto. More specifically, the plate-like rectangular insulating substrate 1 is made of sintered alumina or ceramics, etc. and has a front face 1A and a rear face 1B. It should be noted here that all directional indications such as "front", "rear", "upper", "lower", etc. are based on the illustration in FIGS. 1(c) and 1(d), hereinbelow. On the front face 1A of the insulating substrate 1, a resistor portion or layer 1c of a curved shape, a first electrically conductive portion 1e of a bent shape and a second electrically conductive portion 1g of a linear shape are printed and baked. The resistor portion 1c is formed so as to extend downwardly in a zigzag manner along a left side edge on the front face 1A of the substrate 1 and includes an electrode 1h for the connecting terminal pin 3a, provided at an upper end thereof, a first circular portion 1a, a second circular portion 1b, and an electrode 1i for the connecting terminal 3d, provided at a lower end thereof. The first electrically conductive portion 1e is formed so as to extend downwardly generally in a V-shape on an upper right portion of the front face 1A and includes an electrode 1j for the connecting terminal pin 3b, provided at an upper end thereof and a first central portion 1d which is provided at an lower end thereof so as to be disposed at the center of the first circular portion 1a of the resistor portion 1c. The second electrical conductive portion 1g is formed so as to extend laterally on a lower right portion of the front face 1A and includes an electrode 1k for the connecting terminal pin 3c, provided at a right end thereof and a second central portion 1f which is provided at a left end thereof so as to be disposed at the center of the second circular portion 1b of the resistor portion 1c. Thus, the electrodes 1h, 1i, 1j and 1k are formed at four corner portions of the front face 1A so as to be disposed in a symmetric relation with respect to corresponding four corners of the front face 1A. Since the first circular portion 1a is disposed above the second circular portion 1b, the first central portion 1d and second central portion 1f are provided approximately at the center in the lateral direction of the front face 1A with the first central portion 1d being disposed above the second central portion 1f. The electrodes 1h, 1i, 1j and 1k are formed with through-openings 1h', 1i', 1j' and 1k', respectively. Through-holes 1l and 1m are formed on the front face 1A so as to be, respectively, disposed above the first circular portion 1a, and between the first circular portion 1a and the second circular portion 1b. The electrodes 1h, 1i, 1j and 1k, first central portion 1d and second central portion 1f are provided with electrically conductive paste mainly consisting of silver.
The insulating casing 6 is made of synthetic resin and is formed into a rectangular shape. The insulating casing 6 includes a frontf wall 6H, an upper side wall 6D, a lower side wall 6E, a left side wall 6F and a right side wall 6G with the rear face 6B being not formed with a wall, a rectangular accommodation recess for accommodating the insulating substrate 1 therein, which is enclosed by the front wall 6H, upper side wall 6D, lower side wall 6E, left side wall 6F and right side wall 6G is provided in the insulating casing 6. An upper cylindrical boss 6I and a lower cylindrical boss 6J each provided with a through-hole are integrally formed with the insulating casing 6 on the front face 6A and at the center in the lateral direction of the insulating casing 6. Further, an upper cylindrical projection 6a and a lower cylindrical projection (not shown) each extending over a slight distance from one face of the front wall 6H opposite to the front face 6A in a direction remote from the front face 1A are integrally formed with the insulating casing 6 and are, respectively, disposed at the center in the lateral direction of the insulating casing 6 above the through-hole of the upper cylindrical boss 6I and between the through-hole of the upper cylindrical boss 6I and the through-hole of the lower cylindrical boss 6J. The upper side wall 6D includes a front wall portion 6Da and a rear wall portion 6Db with the front wall portion 6Da being slightly larger, in thickness, than the rear wall portion 6Db. Likewise, the lower side wall 6E, left side wall 6F and right side wall 6G include front wall portions and rear wall portions, respectively, so that a rectangular bearing face 6C for supporting the insulating substrate 1 is formed on side edges of the front wall portion 6Da and the front wall portions of the lower side wall 6E, left side wall 6F and right side wall 6G at the joint with the rear wall portion 6Db, and the rear wall portions of the lower side wall 6E, left side wall 6F and right side wall 6G, respectively.
The conventional high-voltage variable resistors are further provided with an adjusting rod 4 for adjusting the focusing voltage through manual rotation thereof and an adjusting rod 5 for adjusting the screen voltage through manual rotation thereof. The adjusting rod 4 has a knurled portion 4a formed at one end thereof for facilitating the rotation and a flange portion 4b formed at the other end thereof. Similarly, the adjusting rod 5 has a knurled portion 5a and a flange portion (not shown). An elongated first sliding member 2a having one end 2a1 and the other end 2a2 is fixedly attached to one face of the flange portion 4b remote from the knurled portion 4a with the other end 2a2 being disposed concentrically with the axis of the adjusting rod 4. In the same manner as described above, an elongated second sliding member (not shown) having one end and the other end is fixedly attached to one face of the flange portion of the adjusting rod 5 remote from the knurled portion 5a with the other end of the second sliding member being disposed concentrically with the axis of the adjusting rod 5. The connecting terminal pins 3a for receiving the high voltage supplied from the flyback transformer, 3b for supplying the focusing voltage, 3c for supplying the screen voltage, and 3d for earthing are, respectively, fitted into the through-openings 1h', 1j', 1k' and 1d' of the insulating substrate 1 so as to extend therethrough and then, are soldered to the insulating substrate 1 on the front face 1A. Accordingly, deposited solder portions 9 are formed at the through-openings 1h', 1i', ij, and 1k' on the front face 1A. It should be noted that flux deposited on the insulating substrate 1 during the soldering causes deterioration in electrical insulation of the insulating substrate 1 and therefore, is removed from the insulating substrate 1 by washing. When the adjusting rod 4 for adjusting the focusing voltage and the adjusting rod 5 for adjusting the screen voltage are, respectively, fitted into the through-hole of the upper cylindrical boss 6I and the through-hole of the lower cylindrical boss 6J in a direction from the rear face 6B to the front face 6A so that the other face of the flange portion 4b adjacent to the knurled portion 4a and the other face of the flange portion of the adjusting rod 5 adjacent to the knurled portion 5a may be brought into contact with the one face of the front wall 6H remote from the front face 6A with the sliding members 2a and 2b being, respectively, secured to the flange portion 4b of the adjusting rod and the flange portion of the adjusting rod 5, the knurled portions 4a and 4b project out of the upper cylindrical boss 6I and lower cylindrical boss 6J, respectively. Then, the insulating substrate 1 having the connecting terminal pins 3a, 3b, 3c and 3d soldered thereto are fitted into the accommodation recess of the insulating casing 6 so as to be positioned through fitting of the upper cylindrical projection 6a and lower cylindrical projection 6b into the respective through-holes 1l and 1m so that a part of each of the upper cylindrical projection 6a and lower cylindrical portion 6b may project out of the rear face 1B of the insulating substrate 1. The insulating substrate 1 is fixedly attached to the insulating casing 6 by securing the part of each of the upper cylindrical projection 6a and lower cylindrical projection 6b projecting out of the rear face 1B, through melting thereof, to the rear face 1B with peripheral portions of the front face 1A of the insulating substrate 1 being in contact with the bearing face 6C of the insulating casing 6, whereby the connecting terminal pins 3a, 3b, 3c and 3d project out of the rear face 6B of the insulating casing 6 at right angles to the insulating substrate 1 so as to be electrically connected to the flyback transformer.
Furthermore, since a relatively high voltage is applied to the connecting terminal pins 3a, 3b and 3c, the connecting terminal pins 3a, 3b and 3c are, respectively, partially protected by cylindrical insulating covers 7a, 7b and 7c. It is to be noted that, since a voltage applied to the connecting terminal pin 3d for earthing is relatively low, the connecting terminal pin 3d is not required to be protected by an insulating cover.
Subsequently, thermosetting resin 8 such as epoxy resin, etc. is applied to the whole surface of the rear face 1B of the insulating substrate 1 up to the rear face 6B of the insulating casing 6 for the purpose of electrically insulating the insulating substrate 1, absorbing an impact to be applied to the connecting terminal pins 3a, 3b, 3c and 3d and fixing the insulating covers 7a, 7b and 7c in position. Accordingly, thickness of the thermosetting resin 8 is required to be sufficiently large therefor.
In the above described arrangement of the prior art high-voltage variable resistor, the one end 2a1 of the first sliding member 2a fixedly attached to the flange portion 4b of the adjusting rod 4 is caused to make a circular motion along and in sliding contact with the first circular portion 1a of the resistor portion 1c upon manual rotation of the adjusting rod 4 with the other end 2a2 being in contact with the first central portion 1d of the first electrically conductive portion 1e. Likewise, the one end of the second sliding member fixedly attached to the flange portion of the adjusting rod 5 is caused to make a circular motion along and in sliding contact with the second circular portion 1b upon manual rotation of the adjusting rod 5 with the other end of the second sliding member being in contact with the second central portion 1f of the second electrically conductive portion 1g. Accordingly, a high voltage supplied from the flyback transformer to the connecting terminal pin 3a is lowered by a part of the resistor portion 1c extending from the electrode 1h to the first circular portion 1a and then, is varied upon manual rotation of the adjusting rod 4 so as to supply the focusing voltage from the connecting terminal pin 3b owing to electrical contact of the first circular portion 1a of the resistor portion 1c by the first central portion 1d of the first electrically conductive portion 1e through the first sliding member 2a.
The focusing voltage is further lowered by another part of the resistor portion 1c extending from the first circular portion 1a to the second circular portion 1b and then, is varied upon manual rotation of the adjusting rod 5 so as to supply the screen voltage from the connecting terminal pin 3c owing to electrical contact of the second circular portion 1b of the resistor portion 1c by the second central portion 1f of the second electrically conductive portion 1g through the second sliding member.
However, the known high-voltage variable resistors have such a disadvantage that, when the connecting terminal pins 3a, 3b, 3c and 3d are subjected to a large bending moment, the deposited solder portions 9 tend to be separated from the insulating substrate 1, resulting in faulty electrical conduction between the connecting terminal pins 3a, 3b, 3c and 3d and the insulating substrate 1. Meanwhile, even if it is so arranged that a force applied to the connecting terminal pins 3a, 3b, 3c and 3d is absorbed by the thermosetting resin 8, it becomes necessary to inject a greater amount of the thermosetting resin 8 than required for electrical insulation.
Furthermore, the prior art high-voltage variable resistors have such an inconvenience that, if flux used for soldering of the connecting terminal pins 3a, 3b, 3c and 3d to the insulating substrate 1 is not completely removed from the insulating substrate 1 by a washing process and remains on the insulating substrate 1, creeping discharge undesirably takes place on the front face 1A of the insulating substrate 1.
Moreover, the conventional high-voltage variable resistors have been disadvantageous in that, unless a strict control over materials of the solder is exercised, an undesirable phenomenon that dewetting of the solder is caused through diffusion, in the solder, of silver contained in the electrically conductive paste takes place at the electrodes 1h, 1i, 1j and 1k provided with the electrically conductive paste, thus resulting in faulty soldering.
The conventional high-voltage varible resistors have such an disadvantage that, since the insulating casing 6 or the insulating covers 7a, 7b and 7c are deformed by heating of the thermosetting resin 8 for curing thereof, the connecting terminal pins 3a, 3b, 3c and 3d are not positively held by the insulating casing 6, thereby causing displacement of the connecting terminal pins 3a, 3b, 3c and 3d.
Furthermore, although thickness of the thermosetting resin 8 is required to be sufficiently large as described above, the thermosetting resin 8 has a low thermal conductivity of approximately 10.sup.-3 cal/cm.sec...degree.C., so that heat produced at the resistor portion 1c is not emitted out of the insulating casing 6 if the thickness of the thermosetting resin 8 is quite large and thus, temperature in the insulating casing 6 rises considerably, thereby resulting in deterioration of elasticity of the thermosetting resin 8 and sliding members 2a and 2b or degradation of the resistor portion 1c. Accordingly, the known high-voltage variable resistors have such an inconvenience that since load applied to one unit area of the insulating substrate 1 is restricted to some value, the insulating substrate 1 is required to have a large area if a high-voltage variable resistor for large electric currents is to be produced.